1、explain
2、基础使用
在使用explain关键字时，只需要在所执行语句前加上explain即可
mysql> explain select * from stu where id=3;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | stu | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
3、id
4、select_type
5、type
（1）当根据主键or唯一索引进行访问时，type参数会显示为const：
mysql> explain select * from stu where id=3;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | stu | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
（2）当不查询任何表时，type参数显示为NULL：
mysql> explain select 'a';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
（3）使用非唯一性索引，type参数通常显示为ref：
6、possible_key
显示这张表上可能用到的索引，一个或者多个
7、Key
实际使用的索引，如果为NULL，则没有使用索引
8、Key_len
表示索引中使用的字节数，该值为索引字段最大可能长度，并非实际使用长度，在不损失精确性的前提下，长度越短越好
9、rows
MySQL认为必须要执行查询的行数，在innodb引擎的表中，是一个估计值，可能并不总是准确的。
10、filtered
表示返回结果的行数栈读取行数的百分比，filtered的值越大越好
